The microcomputer system of a taximeter consists essentially of an arrangement of a microprocessor with memories, input-output elements and supply modules which are combined as a system and which can decide, compute, display and store data by means of a predeterminable program based on the input of distance and/or time signals. A much-accepted type is the MOS microprocessor (for instance Intel 8049) which works, for instance, with an eight bit word length compared to the bipolar embodiments. As a part of the microcomputer system, the microprocessor or CPU serves for the processing of data according to a predeterminable rule. The directions for the latter are stored in a read-only memory or ROM as thus identified instructions and they represent the internal working program of the system. Information is fed one time into the read-only memory (ROM) and remains there permanently. The function of the ROM is limited so that the memory content can be read out. In the case of this application, the ROM serves primarily for program storage. However, during each change of information, the ROM must be exchanged for a new ROM which is programmed with correspondingly changed data. In the case of taximeters, the tariff data which form the basis for calculation of a fare are also subject to such changes. In known taximeters, one supplied the changed tariff data into a separate, exchangeable, programmable ROM (PROM) and, at the appropriate point in time when the new tariff became effective, it was exchanged for the present module (PROM). Of course, considerable effort is expended in this mode of operation when the tariff changes frequently.
In addition, a write-read memory (RAM) for the processing of variable data is provided in the microcomputer system. Since information can be read in the RAM and again read out at random in an advantageous manner, this type of memory is extremely suitable for the storage of data or the intermediate storage of data, however, care must be taken that the feed voltage is not lost because the information may be falsified or may be lost due to a voltage breakdown.
It is therefore absolutely necessary in the storage of information in write-read memories (RAM) which are constructed as volatile memories, to provide suitable measures which activate a protective device in dependence on power supply failures or the decrease of supply voltages below a predetermined value and these protective devices guarantee that the information remains in the RAM.
A known circuit arrangement for bridging power supply failures consists in that, for instance, a buffer battery is assigned to the RAM area which saves the stored information temporarily. However, it has been found that, in addition, for instance during power supply failure, due to capacities or inductances in the computer system or the power supply part, undefined voltage conditions result which may falsify information stored in the write-read memory (RAM). In addition to protecting the supply voltage for the RAM area by means of a buffer battery, also for protection of the information in the RAM, an additional measure is necessary which prevents all undefined voltage pulses which result during power supply failure in the system and which may act as interference pulses from affecting the RAM area.
In this connection, according to the German Patent No. 28 03 202, a circuit arrangement for the protection of data during power supply failure or decreasing supply voltage is specified for information stored in write-read memories (RAM) which are constructed as volatile memories. As measures for protection of information in the RAM during voltage failure or decrease of the supply voltage below a minimum value, a voltage monitoring circuit is provided which, in the case of protection, i.e., only during voltage decrease, takes over the supply of the RAM area with operating voltage from a charge storage or long-term storage of a decoupling network. The decoupling network then also controls a RAM write blocking circuit whereby any change of information in the RAM area is prevented before any interference pulses are present. In summary, this known device serves exclusively for protection of information in the RAM during voltage failure of the computer system.
During the normal operation of a system, i.e., during correct operating voltage conditions, the known system does not offer adequate protection, for instance, for tariff data and/or control data in an RAM of a taximeter since, due to any system interferences or manipulations, it would be conceivable to feed erroneous information into the RAM.
It is an object of the invention to provide a circuit arrangement for the permanent protection of data, particularly of tariff data and control data, in a volatile write-read memory (RAM) of a taximeter, wherein the measures for protection can only be cancelled by means of an authorized action in the circuit arrangement.